Wireless communication systems are each made up of component terminals communicating with one another wirelessly. One such system is the wireless LAN (local area network) gaining widespread use by replacing conventional wired LANs. There exist a number of standards for the wireless LAN. The IEEE 802.11 standard stipulated by a working group under the IEEE (Institute of Electrical and Electronics Engineers) 802 standardization committee is known in particular for its provisions regarding the physical layer and a MAC (medium access control) sublayer (data link layer), the upper layer closest to the physical layer.
With respect to the usage of the medium characterized as “wireless”, the IEEE 802.11 standard divides the physical layer into two sublayers: a PMD (physical layer dependent) sublayer and a PLCP (physical layer convergence protocol) sublayer. Depending on the properties of the medium, the PMD sublayer stipulates a plurality of transmission systems including the frequency hopping system, direct-sequence spread-spectrum system, and infrared intensity modulation system. The PLCP sublayer constitutes a protocol for communicating physical layer information. Under the protocol, a PLCP header is provided to retain such information as the modulation method in use, transmission rate, and data length.
The data link layer that exists above the physical layer is composed of the MAC sublayer and an LLC (logical link control) sublayer. The IEEE 802.11 standard covers the layers up to the MAC sublayer, the upper layer closest to the physical layer. The MAC sublayer stipulates control over access to the medium, with a MAC header arranged to hold such information as a medium use reserved time and a device address. A MAC frame under the MAC sublayer is encapsulated into a PLCP frame under the PLCP sublayer before being transmitted.
In the above wireless communication system, each of the configured wireless terminals works on batteries and is expected to operate in less power-consuming fashion than before. In fact, in a wireless LAN setup, terminals tend to consume more power because communications take place whenever data sent from an access point or any one terminal is received by each terminal. A technique has thus been proposed to ease that tendency toward higher power dissipation. The technique involves having each terminal check the destination address of any received packet (frame) to determine if the packet is destined for this terminal. If the packet is not found addressed to the terminal, then the terminal is arranged to halt the subsequent receiving procedure. For example, an error detection code for use in a header of a layer higher than the physical layer is prepared and attached to each packet. Upon receipt, the code-carrying header is checked by the terminal for any error. If no error is detected, the destination address and a packet duration time held in the header are used by the terminal for transition into power-saving mode (e.g., see Japanese Patent Laid-open No. 2000-261462, FIG. 1).